Lecture 8

Question 1

Explain the birth of the EcochG

Answer 1

• One year after Berger’s discovery of EEG, the first auditory evoked response was recorded from the auditory nerve of a cat (Wever & Bray, 1930)
• It likely wasn’t a neural response – it was the CM
• All early work with EcochG was invasive

Question 2

Explain the different parts of the EcochG

Answer 2

• CM is following the fluctuations in the stimulus
• CAP is the little burst of activity in the beginning
• DC is the pedestal that the whole thing is riding on

Question 3

Explain the different responses of the EcochG

Answer 3

• Transient Response (not cochlear)
  
  • CAP (wave I of ABR)… also called N1 (first negativity)
• Sustained Response (cochlear)
  
  • SP (.5-1 uV)
    DC shift (basilar membrane motion)
• Steady-State Response (cochlear)
  CM (< .5 uV)

Question 4

Electrocochleography isn’t all ____

Answer 4

Cochlear

Question 5

____ and ____ of the ABR are the same thing

Answer 5

CAP, wave I

Question 6

What is the closet we can get to measure an EcochG without being invasive?

Answer 6

We can get close enough to measure these in humans to not be invasive (just to the edge of the far field of the auditory nerve in the ear canal)

Question 7

Explain endolymph

Answer 7

• Endolymph is different from all other extracellular fluids
• A very high concentration of potassium ions and low concentration of sodium ions
• Maintained by ion pumps in the vascular (and metabolically demanding) Stria Vascularis
• Very POSITIVE relative to the perilymph (+80 mV)

Question 8

Explain the intracellular fluid

Answer 8

Normal intracellular fluids also high in potassium but NEGATIVE relative to extracellular fluids (a function of the permeability of the membrane to sodium (about –50 mV)

Question 9

What is the potential difference?

Answer 9

Therefore, the potential difference is roughly 130 mV—this is the endocochlear potential (i.e., the battery that powers hearing)

Question 10

What happens when the tiplinks open?

Answer 10

• When the tip links open the ion channels, potassium flows into the inner hair cells, depolarizing the membrane
• Repeatedly as the basilar membrane moves back and forth

Question 11

What does the sum of the fluctuating fields produce?

Answer 11

• The sum of these fluctuating fields produce the cochlear microphonic—it is the receptor potential of the cochlea
• Predominantly reflecting OHCs

Question 12

When you lose OHC you lose ____

Answer 12

CM

Question 13

CM is primarily ____

Answer 13

OHC

Question 14

What is the summating potential?

Answer 14

• This is a DC shift that occurs while the stimulus is present (a sustained potential)
• Primarily activity at the IHC synapse
• Negative for most stimuli, but can be positive for high-frequency tones when measured from the promontory

Question 15

What is the difference between CAP and wave I

Answer 15

Difference between CAP between wave I is that CAP goes negative

Question 16

What are the 3 recording approaches?

Answer 16

Easiest approach
1. Record in the ear canal

Harder approaches
2. Tympanic membrane electrode (put an electrode on the TM)
3. Transtympanic electrode (put a needle through the TM on the promontory, which is between the oval and round window)
- In the near field

Question 17

Trans-tympanic measurement (TT)
-what is the size of CAP compared to ABR?

Answer 17

• Largest responses (e.g. CAP may be 5-10 µV when recorded trans-tympanically)
• Where as in the ABR the CAP is 0.2 uV

Question 18

Tympanic membrane electrode (TM)

Answer 18

• These are the ones that go in the cotton ball, soaked in electrolyte fluid, and put it against the TM (half a microvolt bigger)
• A little bit more invasive than extra-tympanic electrode

Question 19

Extra-tympanic electrode

Answer 19

• Ear canal electrode
• Insert earphone wrapped in foil
• These are a bit more expensive

Question 20

If you subtract alternating polarities to see the ____

Answer 20

CM

Question 21

If you add alternating polarities, you get rid of ____ and are left with ____

Answer 21

CM, SP & CAP

Question 22

____ is the negativity that starts before the CAP and lasts the whole stimulus

Answer 22

SP

Question 23

In the middle of the SP, you have the ____

Answer 23

CAP

Question 24

When we use the click, there is no ongoing ____

Answer 24

SP (can only see the CAP)

Question 25

Can you always see the SP?

Answer 25

- Sometimes the SP is difficult to identify - Solution: raise the stimulus rate - The receptor potential is not reduced at high rates - Compound action potential is reduced at high rates

Question 26

What doesn't respond to ABR at high rates?

Answer 26

Wave I

Question 27

What needs a slow rate to be seen?

Answer 27

CAP

Question 28

What 4 reasons do we measure the EcochG?

Answer 28

1. Wave I enhancement (for neurodiagnostic) 2. Assessment of severe-profound loss (ABR won’t work on those with severe-profound loss) 3. Ménière’s 4. Synaptopathy/neuropathy

Question 29

What does near field recording allow for?

Answer 29

- Can be recorded without averaging - Enhancement of Wave I of the ABR - Wave I is useful for identification of tumours

Question 30

EcochG can pull ____ out (sometimes labelled N1)

Answer 30

Wave I

Question 31

How do you assess severe-profound loss?

Answer 31

- CM can occur with severe-profound hearing loss - Suggests OHC function (but useless if no IHC function) - SP may be reduced or absent with cochlear hearing loss - CAP tends to follow audiogram - ABR generally only to moderately-severe

Question 32

What can the TT CAP be used for? Why?

Answer 32

- TT CAP can be used to estimate hearing thresholds at levels that are beyond ABR—even TM CAP can often do this - Why? because behavioural–physiologic differences are close to 0 (whereas they are ~30 dB for ABR) - But standard deviations are still on the order of 10–15 dB (same as ABR) - This is useful for infant cochlear implant candidates

Question 33

Can get wave I right down to threshold when doing ____

Answer 33

TT CAP

Question 34

With a lot of HL (severe), you can get TT CAP where you would not get ____

Answer 34

ABR

Question 35

Match between ____ and TT CAP

Answer 35

Behavioural threshold

Question 36

Explain the diagnosis of meniere's

Answer 36

- The SP is often large in Ménière’s (or in other cases of endolympatic hydrops) - Possibly mechanical diplacement of BM, or metabolic disturbance (Eggermont) - This will diminish as hair cells die - The CAP tends to be smaller - The SP/CAP ratio is often enlarged

Question 37

What is the downside of meniere's diagnosis?

Answer 37

Downside of Meniere's diagnosis with EcochG is that Meniere's fluctuates

Question 38

____ tracks meniere's very well

Answer 38

SP/CAP ratio

Question 39

SP/CAP Ratios (what is abnormal)?

Answer 39

- Generally, anything above .4 is unusual (note specificity) - This ratio is consistent for males and females, and for different electrode montages - It does vary as a function of CAP level though!

Question 40

Auditory Synaptopathy / Neuropathy

Answer 40

- In Ménière’s, the SP is enlarged but the CAP is not - so the SP/CAP ratio is large - In synaptopathy/neuropathy, the active mechanisms in the cochlea are intact (e.g., OHCs), so the SP (OHC & IHC) and CM (OHC) should be less impacted than CAP (post-synaptic) - The SP/CAP ratio should still be large, not because of an abnormally large SP, but because of an abnormally small CAP - Although enlarged SP has been found in synaptopathy! - Reasons are not understood

Question 41

CAP/SP ratio of less than ____ provided a sensitivity and specificity ≈ 80–90%

Answer 41

1

Question 42

Etiology of neuropathy

Answer 42

- Loss of sensory cells (IHCs) or presynaptic deficiency - Summating potential and CAP affected - Post-synaptic or neural deficency - Summating potential intact, SP/CAP ratio large (or CAP/SP ratio small)

Question 43

EcochG measurement paramters - filter settings

Answer 43

5 (SP)- 30 (CAP) Hz  1500 for SP and AP

Question 44

EcochG measurement paramters - # of averages

Answer 44

100-200 for TT 1000-2000 for ET

Question 45

EcochG measurement paramters - analysis window

Answer 45

5-12 ms for click, longer for tone burst

Question 46

EcochG measurement paramters - stimulus polarity

Answer 46

Alternating to remove CM

Question 47

EcochG measurement paramters - stimulus rate

Answer 47

24/sec max (better < 10/sec)